The toggle was intentionally removed for NVidia users, because of a very severe driver bug that reportedly blocked a lot of people from even logging in after they enabled HDR. You can enable it again by putting KWIN_DRM_ALLOW_NVIDIA_COLORSPACE=1 into /etc/environment and rebooting
Note that you may experience that driver bug as well; if you do, removing that environment variable and rebooting should allow you to log in again.